In preparing nylon 6 yarns on a commercial basis, it has been the practice to spin and collect the filaments in a single step operation. In a separate second step, the filaments are drawn and this step may be combined with a third step wherein the drawn filaments are bulked. The resulting yarn products are generally uniform in their dye acceptance and are used in carpets, clothing, etc. The two-stage operation of spinning and subsequently drawing is known in the trade and will be referred to herein as the "split process."
To lower costs and increase the rate of production of nylon 6 yarns, the spinning and drawing steps have been combined so that the filaments are spun, cooled, and drawn in a single continuous operation. While the mechanical processing and handling of the filaments resembles that of the split process, the resulting filaments have been found to vary in dyeability to the extent that carpets and other articles prepared therefrom appear to be "streaky."
The non-uniform dyeability of the filaments is believed to result from the presence of both hexagonal (beta- ) and monoclinic (alpha- ) crystalline forms of nylon 6. In the "as spun" state prior to drawing, the filaments contain large amounts of beta crystallinity, but there is a strong tendency for the beta form to convert to the more thermodynamically stable alpha form. This conversion can be accomplished, for example, by allowing the filaments to stand in the presence of moisture or by subjecting the filaments to relatively high temperatures.
It is believed that conventional split processing produces filaments of relatively uniform dye acceptance because following spinning the filaments are customarily treated with an aqueous-based finish emulsion and are then taken up in package form. The yarn packages are allowed to stand, generally from 6 to 12 hours before the filaments are drawn, and during this time, the aqueous finish emulsion has ample time to penetrate the filaments so that when drawn, the filaments contain a high percentage of alpha crystallinity and appear to dye uniformly.
Because the newer spin-draw-winding processes are continuous and the filaments are not taken up in package form or allowed to stand prior to drawing, there is little opportunity for the finish emulsion to penetrate into the filaments and, as a result, the drawn nylon 6 filaments are believed to consist predominantly of the beta crystalline form. Upon further treatment of the filaments as in bulking and dyeing, the amount of alpha crystallinity is increased and the resulting filaments contain a mixture of alpha and beta forms. Because these forms vary in their ability to accept dyes, the filaments also show a lack of dyeing uniformity.
One method of overcoming the dyeing problem is to treat the filaments at high temperatures (e.g., 200.degree. C.) prior to, and during, drawing. The disadvantages of this technique, however, are that essential elements of the finish emulsion are "fumed" off and the wear factor for various mechanical elements, such as the winding godets, is greatly increased.
What is needed is a method whereby spin-draw-winding of nylon 6 results in a predominantly large amount of alpha crystallinity which promotes relatively uniform dye acceptance. Additionally, to overcome the disadvantages of the prior art, the method must be quick so that it can be used in continuous spin-draw-winding processes and, the method must not require elevated operating temperatures.